IMDB's not always the most accurate source. The Lion In Winter seems to have the same rights situation as The Graduate & The Producers. They were all produced around the same time by AVCO Embassy Pictures. They were all initially released on DVD in the US by MGM. While Studio Canal has the UK rights to The Graduate & The Producers, and Rialto list both in their catalogue, at minimum the home video rights are still with MGM in the US (who licensed The Graduate to Criterion & The Producers to Shout Factory).

Twilight Time stated their option to release it was through MGM. It's unlikely that MGM doesn't have the rights anymore since they still have the rights to The Graduate & The Producers. Twilight Time could still have their license, or if they don't, then MGM could possibly license it to Criterion.